This invention generally relates to means for making semiconductor elements, and more particularly, to an improved process and apparatus which will provide higher yields of semiconductor wafers.
Semiconductor elements have a wide range of applications in the electronics industry. They are used, for example, in rectifiers, transistors, diodes, solar batteries, integrated circuits and the like. The term--semiconductor element--is generally accepted throughout the industry and is intended herein to generically include semiconductor devices and parts thereof formed of host-substrates comprising elements, alloys and intermetallic compounds of silicon, germanium, silicon/germanium, etc. Such semiconductor elements can be of any convenient or suitable shape or form, but are typically used in the form of circular wafers or disks. For purposes of the present invention semiconductor elements will hereinafter be referred to as--wafers.
To impart the needed electrical rectification properties, wafers have active impurities or conductivity modifiers incorporated within the host substrate by high temperature diffusion of vaporized dopant atoms. The doping process involves placing wafers into high temperature processing furnaces equipped with relatively long cylindrically shaped quartz lined heat treating chambers or process tubes where the conductivity modifiers are diffused into the wafers through selectively-defined openings in masks formed adjacent to wafer surfaces. Other high temperature semiconductor device fabrication processes are performed in furnace process tubes including chemical vapor deposition, oxidation, annealing and cleaning, to name but a few.
In manufacturing processes it is especially important to avoid contaminating the semiconductor wafers with undesirable dust-like impurities in order to maximize product yield with the desired electrical properties. As part of the manufacturing process, wafers are customarily placed on transporting "boats" usually consisting of quartz sleds plus paddles which may also include various types of wheeled carriers, trucks, positioning racks, etc. Such boats are used to hold wafers in process tubes during firing operations and are part of the loading system.
The transporting and handling of wafer boats by operators increases the risk of exposure of wafers to undesirable impurities and physical damage, resulting in reduced yields. The process of loading and unloading boat carriers, for example, necessitates moving the boats to a separate transfer station where the wafers are transferred from cassettes to boats which are then transported to the furnace area where they are then placed on the furnace loader for thermal treatment e.g. doping. Subsequently, the boats are returned to the transfer station where the wafers are returned to their cassettes for further processing. Thus, use of wafer boats leads to further contamination outside the furnace through greater exposure to dust, particle pick-up and risk of damage through increased handling.
Wafer boats also generate undesirable contaminants inside furnace processing tubes. Boats per se are a source of undesirable contaminating impurities because they undergo devitrification during thermal cycling and exposure to various furnace ambients. In addition, during push/pull cycles, loading systems deliver and remove wafer loaded boats to and from process tubes. Because of process tube diameter limitations the use of wafer boats creates close tolerances with tube side walls. Consequently, boats and their transport during the push/pull loading and unloading cycles not only generate abrasion dust and particle pick-up, but also shortens process tube life expectancy due to premature groove formation and crystallization of fused quartz.
Efforts by others to reduce particulate contamination through improved wafer handling methods and systems have not been totally satisfactory. For example, one wafer handling system employs dual cantilevered aluminum oxide or silicon carbide ceramic rods sleeved with fused quartz tubes. Although this handling system apparently reduces some unwanted particulates by avoiding contact with process tube side-walls, the system calls for mounting wafers onto boat carriers which serve as a source of potential contamination both outside and inside the furnace. Furthermore, aluminum oxide rods tend to degrade after several thermal cycles, exhibit poor thermal shock characteristics and have a tendency to permanently sag or deflect. Should aluminum oxide rods make contact with fused quartz they can react and undergo mechanical degradation. This system is described in Semiconductor International, pages 150-155, April 1983.
U.S. Pat. No. 3,923,342 discloses an automated wafer conveying device which relies on so-called "air slides" for loading wafer boats. The boat is formed of four slotted quartz rods which collectively support multiple wafers in an upright vertical position. The rods forming the boats are not supported in cantilevered fashion, but instead are linked together as wafer carriers by means of "end quartz rods".
U.S. Pat. No. 3,951,587 suggests high purity sintered silicon carbide made impervious to gases by impregnation with high purity silicon. Their diffusion furnace parts, including the process tube or liner or both are fabricated from silicon/silicon carbide. However, the patentees also suggest instead of eliminating boats and other types of kiln "furniture" that they too be fabricated from silicon/silicon carbide.
U.S. Pat. No. 3,604,694 suggest heat treatment of silicon disks for doping in a horizontal quartz tube which serves as a disk support. Multiple silicon rods line the quartz tube preventing the disks from making contact with the tube. The support rods, however, make contact with the interior walls of the quartz tube. Frictional contact between the rods and quartz tube can serve as a source of unwanted particulate contamination. Accordingly, there is a need for an improved method and system for handling and delivering semiconductor wafers which will provide a lower level of particulates directly and indirectly attributed to the use of paddles, wheeled carriers and other types of wafer boat carriers.
The present invention relates to a system comprising an improved process and apparatus for reducing the level of undesired contaminating impurities generated during the high temperature thermal processing of semiconductor wafers. The improved high temperature equipment and materials of construction used in this system eliminates the need for sleds, paddles, wheel carriers, trucks, separate positioning racks, holders and other types of boat fixtures ordinarily used in thermal processing of semiconductor wafers. The improved system includes diffusion furnaces and other high temperature semiconductor processing equipment fitted or retro-fitted with polished, high density, high purity, spaced ceramic rods suspended in cantilevered fashion. The cantilevered rods eliminate or minimize surface and frictional contact with furnace process tubes during the push/pull cycles of the process reducing unwanted frictional particulates. Because the rods are slotted and spaced from one another so as to support a multiplicity of wafers in adjacent, vertical position the need for boat carriers and additional handling steps at separate transfer stations customarily required with previous systems, can be eliminated. That is to say, the cantilevered-boat free carrier system of the present invention permits direct loading and unloading of wafers to and from covered storage cassettes at the furance eliminating undesirable additional handling steps by operators and exposure of the wafers to ambient conditions outside the furnace. Thus, the present invention provides a means for achieving higher production yields of useful semiconductor wafers.
In addition to the foregoing, the improved boat-free wafer processing system utilizes polished, strong, high temperature, high purity, creep resistant siliconized-silicon carbide as the preferred material of construction with a longer useful life expectancy which avoids the shortcomings associated with systems based on aluminum oxide or silicon carbide or other refractory materials sheathed with fused quartz.
Thus, it is one principal object of the present invention to provide an improved high yield delivery and handling system for semiconductor wafers.
It is a further principal object of the present invention to provide a process for reducing and in some instances substantially eliminating sources of contamination and risk of damage to semiconductor wafers during high temperature thermal processing operations originating mainly from boat carriers, loading systems, furnace tubes, and through handling by operators.
An additional object is to provide a novel boat-free wafer handling apparatus which is also adaptable for retrofitting to existing diffusion furnaces.
A still further object of the present invention is to provide materials of construction which enable the use of cantilevered construction for direct receipt and support of wafers for thermal processing.
These and other objects, features and advantages of the present invention will become more apparent from the following more detailed descriptions.